Field of the Disclosure
The present disclosure relates generally to laterally-graded doping of materials, such as semiconductors.
Background
Materials can be doped with impurities to imbue them with desired properties. For example, the electrical conductivity of intrinsic single crystal silicon can be increased by introducing impurities that increase the number of charge carriers (i.e., electrons or holes) in the crystal lattice. In this case, impurities that increase the number of electrons are referred to as n-type dopants whereas impurities that increase the number of holes are referred to as p-type dopants.
It is often convenient to introduce impurities into materials using laterally-resolved techniques in which a selected region of a larger bulk is selectively doped by impurities (or other species) that have traversed a selected portion of a larger surface. The dopant concentration will be relatively higher in the regions immediately beneath the selected portion of the larger surface whereas the dopant concentration will be relatively lower or even zero in regions that are laterally removed from the selected portion. Examples of techniques in which dopants traverse a surface include diffusion and implantation. In such techniques, the selected portion of the larger surface can be defined using, e.g., masking techniques, beam focusing, and/or combinations of these and other techniques.
Despite the widespread adoption of these techniques, it remains problematic to laterally-grade the doping of materials to achieve desired properties. For example, although a dopant that traverses a material surface through an opening in a mask may diffuse both vertically and laterally away from the opening into the material (thus producing both a vertical and lateral doping gradients), these gradients are constrained by the physical properties of the dopant/material system. The ability of designers to achieve a particular lateral doping gradient is impaired.
One class of devices that would benefit from an improved ability to achieve lateral doping gradients that are not constrained by the physical properties of the dopant/material system is semiconductor devices (e.g., power MOSFETs, BJTs, JFETs, IGFETs, resistors, etc.). For instance, in many high voltage switching devices, on-resistance (Rdson) and breakdown voltage (VBV) are often competing design considerations. For example, both on-resistance (Rdson) and breakdown voltage (VBV) generally decrease with decreasing length of the drift region. Optimizing a switching device for one necessarily impairs the other.
In the past, buried layers that deplete multiple conduction channels in the off-state have been used to improve the breakdown voltage despite the presence of multiple channels. Also, in vertical devices, vertically-graded doping in the drift region has been used to improve Rdson without unduly impairing VBV.